1. Field of the Invention
This invention relates to alkylation of isoalkanes and catalysts therefor.
2. Prior Art
In line with the global move for environmental protection, there has been considerable improvement in gasoline materials for automotive fuels which are relatively free of environmentally harmful alkenes and aromatics. A notable example of such improved gasoline material is an alkylate rich in highly branched alkane contents. Such alkylates may be derived by reacting an isoalkane with an alkene of 3-6 carbon atoms. This reaction is usually accompanied by objectionable side reactions such as isomerization, polymerization, cracking, disproportionation and so on. To eliminate or suppress these side reactions, it has been proposed to use an alkylation catalyst which comprises a relatively strong acid such as typically sulfuric acid and hydrofluoric acid. However, sulfuric acid is highly corrosive, leading to increased energy required for removal of reaction heat. Hydrofluoric acid is difficult or tedious to handle. These highly acidic catalysts have been replaced by a solid acid catalyst such as zeolite, BF.sub.3 modified zeolite, Lewis acid and/or Bronsted acid supported on an inorganic carrier (such as for example SO.sub.4 /ZrO.sub.2) and chlorinated alumina. However, such solid acid catalysts have a drawback when used in the alkylation of isoalkanes in that higher molar ratios of for example isobutane/alkane are needed to suppress objectionable side reactions, and another drawback in that unsaturated oligomers tend to deposit on the acid site of the catalyst to cause quick catalytic deterioration, resulting in frequent catalyst reactivation. Whereas, the use of such a zeolite as comprising a relatively weak solid acid would involve extremely high reaction temperature.
WO94/10106 discloses reacting alkenes with isoalkanes to produce alkylates in the presence of a catalyst comprising an organosulfonic acid having at least one C-F or C-P bond and a Lewis acid. The disclosed process is still disadvantageous because of the use of boron trifluoride (BF.sub.3) which is highly toxic and corrosive, hence demanding extreme caution in handling the catalyst.